Tunable liquid crystal optical devices are known in the art, as for example in WO/2007/098602. These optical devices are flat structures having a liquid crystal layer in which the liquid crystal changes its refractive index as a function of the electric field. By applying a spatially modulated electric field, there is provided a spatially modulated index of refraction with the appropriate geometry for the optical device, for example a lens. Such tunable lenses offer the advantage of being thin and compact, a factor that is important in certain applications, such as in the case of mobile telephone cameras. The performance of such lenses can be measured by two important parameters, namely the tunable diopter range and the level of aberration.
The nature of the variability of index of refraction in response to an electric field depends on the properties of the liquid crystal device. In some devices, a non-linear effect can be observed as the liquid crystal molecules begin to align (molecular group alignment being referred to as director alignment) with the electric field from an initial orientation perpendicular to the electric field. When the electric field is essentially homogenous, the non-linearity means that the change in voltage per unit of change in optical property varies over the range of optical property change of the device, but otherwise, this does not impede operation.
It has been discovered that the optical properties of many tunable liquid crystal optical devices are not consistent over their tunable range. Aberration and scattering properties have been observed to increase as the electric field begins to change the orientation of the liquid crystal molecules, with both scattering and aberration becoming less prevalent as orientation increases along the lines of the electric field. Scattering appears as a cloudiness or murkiness in the liquid crystal layer. In the case of a lens, aberrations induce image distortions. It has been found in the case of some lenses, aberration and scattering drop significantly after peak optical power of the tunable liquid crystal lens. This appears to be the case since peak optical power happens while regions of the lens are still in conditions of initial change of orientation of the liquid crystal molecules. Thus, use of peak optical power of a tunable liquid crystal lens to enjoy maximum power and also the largest range of tunability may be less desirable than using a somewhat reduced range that avoids higher levels of aberration and scattering of the lens. In accordance with the present invention a tunable liquid crystal lens operates within a suitable portion of the tunable range having better optical properties. In an electrically tuned liquid crystal optical device, tunable optical parameter level (optical power, steering angle, etc.) control is provided by controlling the electric field. Magnetic field control is also known. Moreover, the electric field is typically proportional to an applied voltage at electrodes, however, the field level may also depend on a frequency of the applied electrical signal.